Objects containing, or consisting of polymers are used in the dental arts for the replacement or restoration of lost tissue, for the improvement of oral function, for aesthetic enhancement, for the correction of tooth or jaw related problems, as well as other applications. They are required to have a precise fit, as well as certain physical, mechanical, chemical and biological properties. The objects need adequate strength, durability, processing accuracy and dimensional stability. They should be highly and appropriately polymerized to improve strength and stability, and they should be chemically inert so as not to constitute a biohazard. They additionally should be able to be processed rapidly and conveniently.
An example of a polymer object used in the dental arts is a composite resin. Most commercial composite resins consist of a resin matrix, an inorganic filler phase and some coupling agents. The resin matrix generally comprises a monomer system, an initiator system and other stabilizers. The monomer system consists of the unsaturated compounds. These compounds generally comprise one or more ester of ethylenically unsaturated carboxylic acids and the adduct of bisphenol A and glycidyl methacrylate, such as triethyleneglycol dimethacrylate (TEGDMA), ethyleneglycol dimethacrylate (EGDMA) and 2,2-bis-[4-(2-hydroxy-3-methacrylyloxypropoxy)phenyl]-propane ether (Bis-GMA) in U.S. Pat. No. 3,066,112 to Bowen. Another class of unsaturated material is urethane dimethacrylates, such as the 1,6-bis(methacrylyloxy-2-ethoxycarbonylamino-2,4,4-trimethylhexane (UEDMA) which is synthesized form 2-hydroxyethyl methacrylate and 2,4,4-trimethylhexamethylenediisocyanate.
The fillers include glasses, ceramics and inorganic oxides, which are generally the oxides of silicon, aluminum zirconium and other transition metals. Some surface treatments, such as silanization or with titanate, is normally employed before the use of the fillers. The fillers commonly have a particle size ranging from 0.04 to 100 microns, and constitute 50 to 80 weight percent of the composite.
Polymerization of these composite resins is usually achieved by free-radical polymerization using either chemical or photo-initiation. These two methods yield relatively incomplete conversion of the unsaturated compounds. The degree of conversion is generally in the 55-65% range. Incomplete conversion reduces the both mechanical and physical properties of the composite resins, and thus, clinical performance. In addition, unpolymerized monomer can be leached into saliva, and can become a biohazard. Thus, increasing the degree of conversion has benefits of improving the physical and mechanical properties of the composite, while improving the biocompatibility of the composite resin by reducing the leaching of uncured monomer.
The physical, mechanical and chemical properties of a composite include strength, stiffness, hardness, abrasion resistance, toughness, coefficient of thermal expansion, biocompatibility, and micro-shrinkage. Most properties are derived from all three basic components of the composite, although some are associated with one of the three constituents. Micro-shrinkage, one of the main shortcomings of composites, is primarily due to the resin matrix. The physical and mechanical properties, such as strength, hardness, stiffness and abrasion resistance, are highly influenced by resin matrix when the fillers and coupling agents are fixed.
Another disadvantage of dental polymers is that they shrink on hardening. This shrinkage compromises the fit, and, in the case of the composite resin, allows for leakage to occur between the composite and the tooth substrate. Although tooth adhesives can compensate somewhat for this shrinkage, bacterial and fluid leakage occurs between the composite and tooth interface, and can lead to diseases of the pulp (the vital organ contained within the central part of a tooth) and recurrent caries. Methods that improve the degree of conversion and reduce shrinkage would be very advantageous.
Improvements in the properties of polymer-containing materials can be obtained by using different processing methods. Composite cure can be enhanced by intensive visible-light exposure, as is done in the Triad device, (Denisply, USA) or by pressure and heat curing (Ivoclar, Schaan, Liechtenstein). These improvements still result in incomplete and less than satisfactory polymerization, as well as varying degrees of micro-shrinkage. Furthermore, improvements which substantially increase the degree of conversion generally require a laboratory step. Thus, in the dental arts, a highly precise replication of the body tissue is made, a replica is made, and an object is prepared using that replica. This requires two visits. The two visits and the laboratory procedure which can be costly constitute further disadvantages.
Microwave energy has a utility in the processing of polymers. Interest in microwave/radio frequency heating has increased over the last 30 years because of the continuous development of equipment capable of operating in an industrial environment. Conventional heating is concerned primarily with a relatively high-temperature heat source interacting with a relatively low-temperature product surface. In the absence of evaporation or other change of state, the rate of heating and temperature distribution from the surface inwards is governed by the thermal conductivities and specific heats of various constituents of a material. Usually, the rate of heating is slow, so that heat transfer by thermal conduction minimizes the temperature differences that would otherwise exist because of different specific heats; therefore, a relatively uniform temperature gradient from the warm surface to the cooler center of the material is found. In conventional heating, the usual thermal properties such as specific heat, thermal conductivity, coefficient of expansion and emissivity do not change significantly over the usual temperature range of the process.
The main advantages provided by microwave energy include: (1) good penetration, fast heating rates and shorter curing time, resulting in a reduction of the distortion; (2) minimal thermal lag and thermal gradients, which result in a more homogeneous cure and better mechanical properties.
Microwave curing of composites under pressure is one way of reducing polymerization shrinkage. Microwave curing of composites while injected into a mold further reduces porosity, and enhances density, and consequently improves the survival of the dental restoration.
Another problem caused by the residual monomers in the composite is the leaching of the unbound materials. The leaching has an impact on both the structural stability and biocompatibility. The residual monomers are eluted into salivary fluids and brought into contact with mucosal tissues; or be extracted into dentin and diffused to pulp. The elution decreases with the higher degree of conversion. An increase of degree of conversion will result in improved mechanical properties and biocompatibility of composite.
A further example of a polymer object is soft denture liners. Several kinds of soft denture liners are used, these being polysiloxane, polyurethanes, plasticized polymethacrylates, polyvinyl chlorides and polyphosphazene fluoroelastromers.
Most soft-liners have inherent disadvantages. These include the leaching of potentially harmful bonding agents, such as epoxy and urethane adhesives, sulfuric, perfluoroacetic acid; poor adhesion to the polymethylmethacrylate (PMMA) denture base due to the chemical dissimilarities between liners such as hydroxyl-terminated polydimethylsiloxane and PMMA; porosity in denture base and the liner resulting from vaporization of the solvent; dimensional changes caused by micro-shrinkage, or dehydration and rehydration steps.
The improvements of denture liners may be based on the use of novel materials, such as acryloxy or methacryloxy polydimethylsiloxanes and acryoxyalkyl or methacryloxyalkyl-terminated polydialkylsilozanes which have been recently introduced. Since these organosilicons have similarities with PMMA, the bonding between liners and the denture is improved, and use of bonding agents is avoided. However, the curing time for these liners, is relatively long.
Once hardened, on seating of the denture, the oral tissues are subjected to change and compression. A way to improve the fit of existing dentures is to, retake an impression, and have a denture rebased. This procedure is usually done in a laboratory, and the material characteristics are deficient in a manner similar to the materials that undergo water-bath hardening. In addition, the procedure takes two visits, is more time consuming, factors which add to the cost of the procedure. Chairside relines can be made using chemically-cured polymers, such as methylmethacrylates, polymethylmethacrylates, polyvinyl acrylates, 2,2-bis[(p-2xe2x80x2hydroxy-3xe2x80x2methacryloxy-propoxy)phenyl] propane, triethyleneglycol dimethacrylate, urethane dimethacrylate, or light-cured polymers. However, they have a relatively low degree of cure, are extremely porous since no compression is possible, and often can cause chemical and physical irritation of the oral tissues.
Problems existing in dental objects made of, or containing polymers may be caused by relatively incomplete degree of conversion, micro-shrinkage, and porosity. An increase in the degree of conversion, a decrease of micro-shrinkage, and a decrease in porosity will result in improved performance of these objects. Furthermore, a processing which more rapidly imparts improvements will have a greater utility for the dental profession. This can be achieved by microwave heating.
Microwave heating is uniquely different because heat is generated within the material rather than being generated externally. The dielectric properties that govern the rate of internal heating may vary widely in magnitude among various constituents of a multiphase, multi-component product. Furthermore, they may change very significantly with temperature. Therefore, the temperature distribution at a given time in a microwave/RF heated material will depend primarily on the dielectric properties, specific heats and thermal conductivity""s of the material""s constituents. The thermal conductivity""s of the constituents may tend to equalize the local temperature variations, but often, the rate of heating with microwave energy is so high that internal conduction of heat cannot transfer the accumulated heat throughout the material.
The permittivity characteristics of polymers with or without filler at various frequencies and at various temperatures are published in the literature. Von Hippel presents a table of data at frequencies from 100 to 1010 Hz for various polymers and compounds. Ippen presents, in graphical form, the loss factors of various polymers, blends of polymers and polymers with various fillers as a function of temperature at 3 GHz. The selection of the proper frequency in microwave/RF heating is based on important parameter of the product of relative loss factor, epsilonxe2x80x3r by frequency, f. The power absorption capacity of a material depends mainly on xcex5epsilonxe2x80x3r, f and its geometrical shape. Since the shape is variable, the only way to evaluate the heatability of materials is to examine the product xcex5epsilonxe2x80x3r f.
It is an object of this invention to identify polymerizable microwave-sensitive compositions having primary use in the biomedical field, and in particular, in the dental arts, although the materials can be used elsewhere whenever rapid processing of precise shapes are required.
It is an object of this invention to introduce the compositions into a chamber, whereby hydraulic pressure is used to inject the material into a three-dimensional mold, the mold constituting a replica of a body part, and having an air escape vent.
It is an object of this invention to maintain pressure on the said compositions while the composition is in the mold.
It is an object of the present invention to harden the compositions using microwave energy, delivered to the mold which is contained in a microwave chamber.
It is an object of this invention to perform an in situ (directly in the mouth) hardening of the said polymerizable composition used for the restoration of teeth using a hand-held apparatus placed in the vicinity of the composition, which has been placed in a tooth.